Electron beam instruments, such as electron microscopes, are commonly used in the research and development underlying modern integrated electronic circuits and even in the production lines for them. Over the past decade, the technology in processing semiconductor wafers has been pushed forward in achieving ever smaller features in commercial integrated circuits. Today, 1 .mu.m line widths have become commonplace on a VLSI chip and 0.1 .mu.m line widths are being researched. Needless to say, the resolution of electron beam instruments needs to keep pace with this reduced scale.
Scanning electron microscopes for electronic circuits are preferably operated in the range of 0.5 kV to 2 kV in order to minimize charging effects in the insulating parts of the circuits and also to prevent damage to the fabricated device being tested. To make a meaningful measurement on a line of a given width, a resolution is required of better than 1%, for example, a resolution of 2 nm for a line width of 0.2 .mu.m, which is common in present day developmental research. Operation at 1 kV would be preferred. At the present time, the best commercially available electron microscopes suitable for semiconductor wafer measurements provide a resolution of 20 nm at 2 kV with a thermal emission gun and of about 10 nm with a field emission gun. Thus, the available resolution is unsatisfactory even at the upper end of the usable voltage range. For low voltage operation, the chromatic aberration in available scanning electron microscopes becomes the limiting factor in further reducing probe size because of the unavoidable energy spread of electron sources. The probe size is the focussed electron beam diameter which must be less than the required resolution.
Integrated circuits are almost always fabricated on a wafer. Wafers for research purposes may be as little as 2 inches (50 mm) in diameter for advanced materials such as GaAs. However, production line wafers, particularly for silicon, are significantly larger. Most present day production is performed on 4 and 5 inch (100 and 125 mm) wafers. Some production lines use 8 inch (200 mm) wafers. Wafers as large as 12 inches (300 mm) are forecast. It is preferred that the fabricated wafer not need to be diced in order to be inspected in an electron microscope. Needless to say, obtaining 2 nm resolution within a chamber accommodating a 300 mm specimen presents significant difficulty for an electron microscope. Because of the problem with wafer sizes, the commercially usable instruments are equipped with conventional non-immersion magnetic lenses. Examples of non-immersion lens are disclosed by Wolff in U.S. Pat. No. 3,560,739 and by Shiokawa in U.S. Pat. No. 4,639,597 and in Japanese Laid-open Application 62-229643 (1987). Electron microscope lens are generally cylindrically symmetric with a central bore passing the electrons having a diameter d. Surrounding this bore are one or more cylindrically symmetric magnetic poles. In a non-immersion lens, two axially adjacent magnetically coupled pole pieces establish a single magnetic circuit across a narrow gap between the poles in the vicinity of the bore. The resulting magnetic field is strongest in the gap and is used to focus the electron beam. Once the electrons leave the gap region, they are substantially free from further magnetic confinement.
An immersion lens offers markedly better resolution over non-immersion lens. In an immersion lens, the axial gap between two poles is widened sufficiently to allow the specimen to be placed within the gap. Thus the magnetic field sharply peaks adjacent the specimen. Pawley has recently disclosed in a technical article ("Low Voltage Scanning Electron microscopy" appearing in EMSA Bulletin, volume 18, number 1, 1988 at pages 61-64) a 4 nm probe diameter obtainable at 1 kV with such an immersion lens. However, conventional immersion lenses are designed to limit aberrations in such a way that the bores on both poles and the gap between them cannot be made very large. The resultant geometry makes it impossible to place a large semiconductor wafer into the lens gap. Specimen tilting becomes almost impossible with immersion lenses and there are difficulties of extracting signals for secondary or back-scattered electrons desired for certain types of imaging.
Yet another alternative is to use a single pole piece, such as described by Mulvey at pages 359-490 in a chapter entitled "Unconventional Design of Magnetic Lenses" appearing in the book Magnetic Electron Lenses, edited by P. W. Hawkes (Springer, Berlin, 1982). Other examples are disclosed by Reisner in U.S. Pat. No. 2,819,403 and by Smith in U.S. Pat. No. 2,418,432. In a single pole piece design, the specimen is placed between the electron gun and the lens, the magnetic field of which extends towards the gun through the specimen. Such a design allows large specimen size but other problems arise. For instance, in order to maintain good optics by reducing aberrations, the focal plane must be kept close to the pole face, usually tenths of a millimeter. Since the specimen is placed between the gun and the lens, the specimen thickness is limited and the specimen holder is limited. Specimen tilting becomes impossible. Such a design is thus impractical for a commercial instrument.
Bassett et al have disclosed in U.S. Pat. No. 3,707,628 a conical coil in a magnetic lens. In this lens, the electron beam enter the wide part of the conical coil and exit an axial aperture adjacent the specimen. Japanese Laid-open Application 60-258836 (1985) discloses that a conical coil of this sort is useful for tilting a specimen with respect to the electron beam. Nonetheless, these designs are still considered inadequate.